Following neuromuscular injury or disease, surgical tendon transfer procedures successfully restore lost limb function, improve hygiene, and relieve pain caused by spastic muscles. After successful tendon transfer procedures, patients achieve improved function, avoiding the need for constant nursing care. This proposal is based on studies which established the functional relationship between muscle sarcomere length, joint moment arm, and joint production. The thesis of this proposal is that muscle fiber length dramatically affects what we term the "torque profile" (ie. peak torque, range of motion, and the angle at which peak torque occurs). Our objective is to define the adaptation in muscle fiber length which occurs in mammalian muscle following chronic length change, and to determine its influence on the torque profile. The relationship between sarcomere length, moment arm, and torque will be determined in the mouse extensor digitorum longus (EDL) muscle following chronic length change due to immobilization and following surgical alteration of muscle moment arm. The time course of change in muscle properties and torque profile will be determined at various time intervals following the treatment. In addition, the time course of change in muscle fiber myosin content will be determined using monoclonal antibodies to the various myosin isoforms and mRNA probes of myosin messenger RNA precursors. In addition to the animal experiments proposed, we will measure torque profiles in human subjects following surgical transfer of the flexor carpi ulnaris (FCU) muscle to the extensor digitorum communis (EDC). Because these two muscles have quite different fiber lengths, we hypothesize that the strength change observed in patients postoperatively will represent fiber length adaptation within the FCU muscle rather than FCU fiber area changes. Taken together, these studies will establish a scientific basis for the tendon transfer procedures as well as improve our understanding of musculoskeletal design and plasticity.